1. Field of the Invention
The present invention relates to a cable made of twisted wire pairs, and more particularly, to a cable made of twisted wire pairs that is suitable for use in high-speed data communication applications.
2. Description of the Related Art
In general, wire pairs are twisted to minimize the interference of signals from one pair to another caused by radiation or capacitive coupling between the pairs. When a signal is present on a twisted pair, a state known as xe2x80x9cactive, xe2x80x9d the twisted pair naturally creates an electromagnetic field around it. The electromagnetic field thus generated may induce a signal in other twisted pairs located within the electromagnetic field. Additionally, a field generated by one active twisted pair can interfere with the operation of other active pairs located in close proximity to the first pair. As a result, signals transmitted in one pair may generate xe2x80x9cnoisexe2x80x9d within adjoining pairs, thereby degrading or attenuating the signal in the adjoining pairs. This coupling, known as xe2x80x9ccrosstalk,xe2x80x9d worsens as data transmission frequencies and data transmission length increase.
Various telecommunication systems require communication cables comprising an odd number of conductor pairs. A commonly used cable for such purposes is the twenty-five pair, category five cable. This cable, like other cables, must comply with associated TIA/EIA requirements. Various cable construction techniques have been tried by cable manufacturers in an attempt to pass the power sum near-end crosstalk (NEXT) specification for TTA/EIA twenty-five pair category five cables.
For a plenum product, the use of a filler having a star configuration would not allow the product to pass the UL 910 burn test. This is so because the star filler greatly increases the percentage of combustible plastics when compared to a copper heat sink based upon presently known state of the art materials.
The layout of the pairs of conductors comprising a cable is critical in the cable passing the TIA/EIA power sum NEXT electrical specification. One of the more successful attempts utilized a cable construction having the twenty-fifth pair jacketed and used as a center filler with six quads using two or more different pair lay schemes and one or more different quad lay lengths (L) surrounding the filler. However, the location of the twenty-fifth pair inside the filler causes increased installation times and potential for damage. For example, in cables utilizing such a cable layout, the twenty-fifth pair is prone to damage when stripping off the end of the rather thick filler jacket during installation.
Several different cable constructions have been attempted in the past, including having the twenty-fifth pair pulled straight in between two of the quads, having the twenty-fifth pair placed by the center along with the tube filler, and laying the twenty-fifth pair on the outside of the cable core. However, the cables fail to meet the TIA/EIA power sum NEXT requirements for the twenty-fifth pair. In addition, the cables also failed signal reflection loss (SRL), impedance, and attenuation requirements due to instability in the twenty-fifth pair.
It was also found that the twenty-fifth pair interfered with the pairs in the quads closest to it. The damage to the insulation of the twenty-fifth pair was caused by the twenty-fifth pair being pinched between quads, or being pinched between the quads and the filler, or being pinched between the core and the jacket.
A cable construction involving jacketing twelve and thirteen pairs of conductors together to yield a twenty-five pair cable has also been attempted with limited success. For example, the resulting shape of the cable is not round, thus making it harder to install, specifically with regard to conduit fill.
Twisted pair telecommunication wires are bundled together in large cables. Typically, 50 or more pairs of wire are included in a typical cable configuration near its termination point. However, cables coming out of a central telecommunications location may have hundreds or even thousands of pairs bundled together. In operation, each twisted pair within the cable is utilized for transmitting data as well as for furnishing direct current (DC) power to remote equipment. With signal multiplexing, a single twisted pair may service multiple data signals and multiple end users, reducing the number of individual pairs required for a desired level of service and reducing the distance between an access point and a final subscriber.
Recently, demands upon telecommunication systems have greatly increased. With the explosive growth of the Internet, consumers and telecommunication companies alike are seeking new methods for high speed data transmission. In particular, telecommunication companies and other entities are developing methods for supporting digital communication circuits at increased speed and/or distances than have existed in the past. For example, new methods for supporting digital communication circuits at increased speed and/or distance include, but are not limited to, DS1/1C/2, ADSL, SDSL, HDSL, and VDSL (where DSL stands for Digital Subscriber Loop with A=Asynchronous, S=Symmetrical, H=High Speed, and V=Very High Speed). In addition, telecommunication companies and other entities are developing these new methods for use over the existing telephone wiring infrastructure, which is generally composed of twisted pair wires bundled as cables strung over relatively long distances.
With the emerging deployment of the various high-speed digital transport systems and services, the shortcomings of the existing and deployed twisted pair communications cables are quickly becoming apparent. Emerging methods of supporting digital communication circuits, described above, rely upon using increased data transmission frequencies over long distances. For example, normal voice transmissions transmitted over telephone wires occur in a frequency range from greater than 0 to 4 kHz, while DSL applications typically transmit in a frequency range from greater than 0 to about 100 kHz over distances between 12,000 and 18,000 feet. As can be appreciated, emerging digital communications methods are highly prone to error due to crosstalk between pairs within the cable, between adjoining cables, and from outside interference, especially at the point where the incoming signal is interfaced to transport equipment such as a modem.
Typically, existing twisted pair cables attempt to isolate outside interference and crosstalk by using a common shield within the cable and by grounding the shield at a termination point. Alternatively, if multiple shields are used, existing cables fail to isolate various shields within a cable, such that the multiple shields within a cable electrically communicate with each other, especially after prolonged use. Specifically, if a telecommunications cable includes an overall shield surrounding a unit shield, the overall shield may electrically communicate with the unit shield, or else electrical interaction may occur due to shield shorts for pinholes in any insulation. Moreover, typical telecommunications cables currently in use terminate the overall shield by drawing out a drain wire and simply clamping it to ground. Unfortunately, grounding the drain wire usually causes it to act as an antenna that draws interference into the cable from outside sources.
The present invention is directed to a cable for supporting digital communication circuits and increased speed and/or distances. The cable is constructed from multiple shielded or foil screened binder units where each binder unit includes an even number of twisted wire pairs, along with an additional twisted wire pairs paired with, and encircles a filler material along its length. Thus, the total number of paired conductors is an odd number. The even number of twisted wire pairs is evenly divided into quads or sub-units with each quad having at least four twisted wire pairs. A shield or foil screen encloses the quads of twisted wire pairs, the additional twisted wire pair and the filler material to form a screened binder unit having a pre-selected number of twisted wire pairs. Preferably, each screened binder unit has twenty-five or less twisted wire pairs. An overall core wrap encloses a pre-selected number of screened binder units, and a unit shield is applied over the top of the overall core wrap. A drain wire may be pulled between the unit shield and the core wrap of one or more screened binder units. The shield surface faces inwardly for improved termination to ground. Finally, the entire cable may be enclosed by a jacket or sheath.
In one embodiment of the invention, the filler material has a larger diameter than the additional twisted wire pair, and the filler material is twined with the additional twisted wire pair, so that the filler material causes an air gap to surround any portion of the additional twisted wire pair that is not in contact with the filler material. In another embodiment of the invention, the filler material secures the additional twisted wire pair within a longitudinal groove formed in the filler material.
In a preferred embodiment of the invention, the filler material has a dielectric constant higher than a dielectric constant of air. More particularly, the filler material is selected from at least one of the following: polyfluoroalkoxy,TFE/Perfluoromethyl-vinylether, ethylene chlorotrifluoroethylene, polyvinyl chloride, fluorinated perfluoroethylene polypropylene and flame retardant polypropylene.
Also in a preferred embodiment of the invention, the jacket material includes a dielectric layer. The dielectric layer can be a single or a multiple dielectric layer, with each layer comprising at least one of the following: low smoke zero halogen, polyvinyl chloride, flame retardant polyethylene, linear low density polyethylene, polyvinylidene fluoride, ethylene chlorotrifluoroethylene, fluorinated ethylene propylene, thermoplastic elastomer, and polyurethane.
Each conductor can be a bare copper wire, and each should be insulated with an insulating material having a dielectric constant no greater than about 2.5. Normally, each bare copper wire is between 22 AWG and 24 AWG. The insulating material preferably includes at least one of the following: flame retardant polyethylene, flame retardant polypropylene, high density polyethylene, polypropylene, polyfluoroalkoxy, solid or foamed TFE/perfluoromethylvinylether, solid or foamed fluorinated ethylene-propylene, and foamed ethylene chlorotrifluoroethylene.
In the cable of the present invention, the overall shield is isolated from the unit shields, and each shield may be terminated to ground independently of the other. In this way, the inner binder units are isolated from outside interference, e.g., from other adjacent cables. The shields are also isolated from contacting each other or from contacting individual wires or wire pairs, by the overall core wrap, thereby preventing shorts or signal loss through pinholes in the twisted pair insulation.
Moreover, both the overall shield and the unit shield are applied with the foil side inwardly oriented. This arrangement allows the foil to be folded back over the cable and the binder unit, respectively, and terminated using a simple grounding clamp, rather than by grounding the drain wire as is currently the practice. By clamping the shields instead of the drain wire, shielding performance is enhanced because the drain wires are not able to act as an antenna and draw interference into the cable.
By separating the twisted pair wires into manageably sized binder units, convenience and efficiency of use is enhanced. For example, separate digital services may be provided through each of the binder units based upon the frequency spectrum within which they operate. Alternatively, one binder unit may be used as a xe2x80x9csendxe2x80x9d unit, while an adjacent binder unit may be designated the xe2x80x9creceivexe2x80x9d units. By separating xe2x80x9csendxe2x80x9d and xe2x80x9creceivexe2x80x9d functions between binder units, rather than simply between twisted pairs within a single unit, local crosstalk is minimized, leading to increased transmission distances.
The present invention is also directed to a method for manufacturing the above-described cable. First, the pair of conductors is paired with each other to make an even number of twisted wire pairs. Then, the additional pair of conductors is paired, making the total number of twisted wire pairs an odd number. The even number of twisted wire pairs are then evenly divided into quads or sub-units of at least two twisted wire pairs. The additional twisted wire pair is coupled with and encircles around the filler material along its length. Then, the quads of twisted wire pairs and the additional twisted wire pair coupled with the filler material are extended in parallel to form a cable so the quads of conductor pairs surround the additional twisted wire pair and the filler material. Then, the quads of twisted wire pairs and the additional twisted wire pair coupled with the filler material, usually twenty-five or less pairs of conductors, are enclosed by a shield or foil screen to form a screened binder unit. Next, the screened binder units are stranded to form the cable. Finally, a jacket material surrounds the cable.